In offshore oil and natural gas production, hydrocarbons are delivered by tubular risers from wells on the sea bed to a ship or floating production vessel, the hydrocarbons being directed into a hold of the vessel. When the hold is filled, the vessel is typically uncoupled from the riser and moved toward a transfer location near onshore storage tanks or ground transportation facilities. Since the vessel, when coupled with the riser, will align its fore and aft axis normal to the wave fronts of the prevailing sea, provision must be made to allow for rotation of the vessel about a vertical axis relative to the riser connection. This is required because produced hydrocarbon fluids must be transferred from the riser to the hold of the vessel, which is continuously rotating with respect to the riser due to the wave action of the sea.
Swivels are normally used as a rotating link between the riser and the vessel. These swivels require a seal and generally are under mechanical stresses at all times. The seal integrity and mechanical stresses are troublesome factors, especially in high pressure transfer of the hydrocarbon fluids from the riser to the vessel. Hose or flexible steel piping with rigid, non-sliding pressure-tight connections would be preferred to a swivel with its rotating seals. However, flexible pipe systems, unlike swivels, can only rotate a few degrees in one direction.
In the prior art, there are numerous types of flexible connector devices which allow limited rotatability in either direction. Such devices usually permit rotation up to a small fraction of an arc in either direction. Mechanical and geometrical problems arise in handling the free loops of flexible pipe in the intermediate position between extreme clockwise and extreme counterclockwise rotation. "Maypole" or "Soap Box Steering" configurations of the flexible pipe compound these problems with both tension and torsion loads on the flexible pipe.
Because of the problems associated with conventional connector structures between a fluid riser and a floating vessel which receives fluid from the riser, a need exists for improvements in mechanically coupling such a riser to a movable vessel. The present invention satisfies this need by providing a swivel-less apparatus to provide the coupling between the riser and the vessel as hereinafter described.